1. Field of the Invention
The present invention relates to position control systems; more particularly, position control systems having particular advantages when used in record storage disk recording and player apparatus.
2. Discussion of the Prior Art
Transducer positioning systems, particularly those used with magnetic or optical disk recording and player apparatus, have used a so-called velocity loop for long transducer motions, termed seeks, i.e., seeks traversing a large number of concentric circular record tracks. The velocity positioning servo mode is optimally switched to a track-following positioning servo mode at one-quarter track pitch from a target track. Such a track-following control may be compared to a "stop-lock" positioning control in other applications of positioning control systems. The track-following position servo then positions the transducer to faithfully track or follow the target track. In a subsequent seek operation, the track-following position loop is interrupted to return to either a velocity loop, a second positioning loop, or an open loop "bang-bang" servo for moving the transducer to yet another target track. It has been found that when the inter-track spacing is reduced to obtain higher track densities, then overshoot and track-settling problems become more acute when the servo mode is switched from velocity mode to track-following mode at the one-quarter track pitch from the target track. Accordingly, it is desired to provide for a more reliable and faster transition from velocity to positioning servo mode which results in faithful yet rapid track-following mode for carried signal transducers.
Many optical recorders have a goal of high performance at low cost. Accordingly, a so-called fine servo or fine actuator is carried upon a head-carrying arm which is moved by a coarse actuator. Typically, the fine actuator has high frequency response characteristics and provides for rapid and short distance positioning of the transducer with respect to a track being followed or for moving from one track to a second target track, which may be an adjacent track. The coarse servo which positions a relatively large mass head-carrying head arm, as well as the fine actuator, typically has frequency characteristics for handling the longer moves for optimizing the relationship for top performance between the fine and coarse actuators. The servo systems provide for relative positioning of the fine actuator with respect to the coarse actuator to a central or reference position. Such arrangements have been colloquially called "piggy back" carriage servo systems.
The application of such a "piggy back" carriage system is not limited to disk recorders. Actually, the concept was established many years ago for a pattern following or template-controlled coarse-fine positioning servo mechanism. Such an arrangement enabled higher production rates of a pattern controlled machine, such as welding machines or cutting machines. The carried fine actuator rapidly responds to sharp changes in the pattern such that the welding or cutting operation faithfully follows the guiding pattern template while only overcoming minimal inertia of the pattern controlled machine mechanisms. Gardiner, U.S. Pat. No. 2,717,979 shows such an arrangement. Gardiner teaches that the fine actuator, which Gardiner terms a topping servo, is controlled by the absolute positioning of the pattern template; while the coarse servo (called the main servo by Gardiner) is slaved to (always follows) the positioning of the topping servo. This arrangement means that the rapid responding topping servo controls the pattern controlled machine while the main servo follows the motions of the topping servo for maintaining the topping servo in an optimumal position with respect to the main servo controlled carriage; thereby maximizing the range of operation of the topping servo. The Gardiner positioning servo arrangement is also shown in Meyer, U.S. Pat. No. 4,627,039.
McIntosh et al., U.S. Pat. No. 3,924,268 and Merritt et al, U.S. Pat. No. 4,513,332 show magnetic disk recorders having piggy-back arrangements which are servo position controlled for optimizing the relative position of the fine actuator with respect to the coarse actuator. Simons, in U.S. Pat. No. 3,924,063 shows yet another coarse-fine control wherein the fine actuator is permitted to move over a predetermined minimum distance before a coarse actuator operation is invoked. Van Winkle in U.S. Pat. No. 4,191,981 shows fast and slow servo positioning mechanisms in a magnetic multiple disk recorder in which the slow servo mechanism is slaved to the fast servo mechanism; the latter arrangement is not a piggy-back arrangement.
Coarse and fine actuator controls are also widely found, particularly in magnetic disk recorders, for controlling a single carriage. In many instances, the fine control is a position responsive servo while the coarse or seek control is a velocity responsive servo. In some instances, both servo controls use position responsive controls. Examples of such coarse-fine controls of both types are shown in Svendsen U.S. Pat. No. 4,268,785; Kaser et al., U.S. Pat. No. 4,032,984; Case, U.S. Pat. No. 4,103,314; Sordello, U.S. Pat. No. 3,458,785; and Johnson, U.S. Pat. No. 4,333,117. Coarse and fine controls are also shown for optical recorders by van Rosmalen in U.S. Pat. No. 4,425,043 and by Janssen et al. in U.S. Pat. No. 4,561,081.
Not all positioning systems employ two separate servo mechanism control loops. An example of a single loop control for both track seeking and track-following is shown by Matla et al. in U.S. Pat. No. 4,217,612. Matla et al. employ a single position mode servo which includes an analog summer circuit, which during the track-following mode, has a control input of a reference potential. For track switching or seeking, a track increment generator supplies an input central signal to the summer circuit for actuating the loop to move the head or transducer carriage radially of a disk to another track position. In particular, see FIGS. 1 and 7 in this reference for a single loop controller which not only track follows, but uses the same track-following loop for moving a transducer in a seek operation over a plurality of concentric record tracks.
Newell, in U.S. Pat. No. 2,800,769, shows a gun-directing servo as yet another coarse-fine control. The fine control controls the hydraulic speed gear of the gun directing or laying servo similar to track-following in a disk recorder. A coarse control is responsive to an input signal for supplying a relatively large servo driving signal. This driving signal is coupled to a switch in the servo loop which switches between fine and coarse controls. When the coarse output signal is relatively large, the switch responds to the large signal amplitude to switch from the fine to the coarse control. When the output signal of the coarse control is reduced below a given threshold, then the switch returns the servo from the coarse control to the fine control. The gun-directing servo apparently has a 360 degree rotational range. The fine control positions the hydraulic speed gear within a 21/2 degree error, which is about 0.7% of the positioning range. Based upon this small position error range of the fine control, it is believed that this fine control corresponds favorably to track-following in disk recorders. Another aspect of the Newell arrangement is a rate sensor which supplies a rate signal for assisting in deceleration of the hydraulic speed gear toward the target rotational position.
Optical recorders of both the record disk and record sheet (also termed tablet or chip) type have employed servo positioning wherein the light phase between adjacent tracks of optical indicia are reversed; that is, between adjacent record tracks 1 and 2 a black or opaque line extends for controlling the servo positioning. Between adjacent tracks 2 and 3, a transparent line extends between these adjacent tracks. Between tracks 3 and 4, the opaque line is repeated, etc. Track following uses a grey scale representing a mid point between the opaque and transparent track guiding lines. The reversal relationship of the opaque and transparent lines constitute a phase reversal in positioning control. Such an arrangement is shown by King et al. in U.S. Pat. No. 2,843,841. King et al. shows interrupting track-following mode for seeking to an adjacent track, also termed "track jumping". According to King et al., an open-loop pulse is applied to the positioning servo circuits for moving an optical radiation beam toward an adjacent target track. The open-loop pulse is designed to move the beam just over one-half way between two adjacent tracks. At this time, the phase of the servo circuits is reversed and the open loop pulse terminates. At this time, the track-following servo takes over the control of the radiation beam positioning for moving it to the target adjacent track in a track-following mode. Jensen, in U.S. Pat. No. 3,473,164, uses the arrangement for servo positioning shown in King et al., but uses a different servo control mechanism for moving the radiation beam from one track to an adjacent track. Reversal of the servo phase is also employed by Jensen for jumping the radiation beam to an adjacent track.
Even with all of the above variations of servo positioning of a transducer or work object with respect to a record element (disk, sheet or other work element), it is desired to provide for a more rapid and faithful servo positioning, particularly for data recorders having extremely high track densities. It is desired to provide for a more faithful so-called track "capture" such that the servo mode can switch from seeking to track-following more rapidly and reliably. A problem found in using high track densities is that as the transducer is radially moved slower and slower, the disk run-out caused by eccentricity of the disk with respect to its rotating spindle, causes radial motions of the track with respect to the slow-moving transducer. Accordingly, an engineering problem of faithfully measuring radial speed of the transducer with respect to the tracks (which are relatively radially moving inward or outward, depending upon eccentricity) becomes more difficult as well as faithful counting of the track crossings for precisely determining radial position with respect to the radially moving tracks, becomes more difficult. It is also desired to use a velocity servo loop for the long seeks.